Detection of magnetic beads using a magnetoresistive device together with ferromagnetic resonance

ABSTRACT

A method and apparatus for detecting the presence of magnetic beads is disclosed. By providing both a static magnetic field and a magnetic field that alternates in the MHz range, or beyond, the bead can be excited into FMR (ferromagnetic resonance). The appearance of the latter is then detected by a magneto-resistive type of sensor. This approach offers several advantages over prior art methods in which the magnetic moment of the bead is detected directly.

FIELD OF THE INVENTION

The invention relates to the general field of magnetic field measurementwith particular reference to that of magnetic beads that have beenselectively bound to biological structures and/or large molecules.

BACKGROUND OF THE INVENTION

The invention discloses a methodology for detecting magnetic beads,magnetic particles and magnetic nano-particles by inducing ferromagneticresonance (FMR) of their magnetic moment and by using amagneto-resistive (MR) sensor to detect the magnetic field produced bythe FMR. Binding these magnetic beads or particles to biological orchemical molecules thus enables the presence of these molecules to bedetected. In the form of a binding assay, where a matrix of MR sensorsare patterned, this method can be used to identify the presence of amolecule of interest as well as to quantify its population. This methodcan address many other issues in the prior art that also utilizemagnetic bead labeling and MR sensing, thus making this technologywidely applicable.

Detection of biological or chemical molecules by usingsuper-paramagnetic beads and particles as the labeling component and byusing magneto-resistive sensors for detection of such labels is regardedas a promising technique to achieve accurate molecule counting with aresolution of several molecules or even a single molecule. It has thepotential to enable fast, efficient, and economical biologicalapplications, such as in-field virus and bacteria detection.

Binding assays to detect target molecules is a widely used technique inthe biological, biochemical, and medical communities. The selectivebindings commonly used include polynucleic acid bindings orhybridizations involving RNA and DNA, many types of ligand-to-receptorbindings, as well as antibody to antigen bindings. The target moleculesin these bindings, for example, proteins or DNA, can also be adistinctive component or product of viruses, bacteria and cells, whichmay be the actual objects of interest for the detection.

In a binding assay, the binding molecules are attached to a solidsubstrate as “capture molecules”. When the assay is exposed to aliquid-form sample, where the target molecules attached to a physicallabel are contained, the binding molecules capture the target moleculeswith the selective bindings and immobilize the target molecules on thesurface. This capture process is called “recognition”. The recognitionevents can be made to generate detectable signals from the attachedlabels and, consequently, the presence or absence of a target moleculecan be detected.

In various prior art techniques utilizing the labeled binding process,the labels originally attached to the target molecule are alsoimmobilized on the surface after the recognition process. The labels areeither bound together with the target molecules on the surface(“sandwich” assay) or are by themselves (“competitive” assay). After theremoval of non-bound labels, the bound labels can then be made togenerate measurable signals.

When using magnetic beads or particles as the labeling component and aMR sensor as the detector, the MR devices are embedded below the bindingsurface and are usually covered by a protective layer. When the magneticbeads or particles are bound to the surface above a MR sensor, they cangenerate a magnetic field spontaneously or, in the case ofsuper-paramagnetic beads or particles, if an applied magnetic field ispresent. This magnetic field from the beads or particles can be sensedby the MR device, which can then provide a voltage signal.

The magnetic labels described in published studies or as patents areusually super-paramagnetic beads—nano-particles (or larger beads thatcomprise nano-particles suspended in a non-magnertic matrix) that haveno magnetization at room temperature without an externally appliedmagnetic field because of the super-paramagnetic effect. Such labels aredesired in biological applications because they do not aggregate (atzero field). The beads or particles described in the prior art usuallyrange in size from tens of nanometers to several microns. When thelabels are attached to a surface after the recognition process, eithersingle or multiple labels are attached to each MR device. However, thesensing mechanism has generally been the same for all the previousdesigns. When the magnetic labels are attached to the MR sensor topsurface, the field generated by the magnetic moment of the beads orparticles will either act directly on the underlying MR sensor or it maycancel a portion of the applied magnetic field acting on the sensor. Forsensors that have no attached labels, the magnetic field from themagnetic labels is not present.

FIG. 1 is a schematic representation of the scheme outlined above.Magnetic bead 1 (which term will, hereinafter, be assumed to includemagnetic beads, as well as particles and nano-particles) is attached tothe MR sensor surface by the binding pairs 5 after recognition. The MRsensor used or referred is usually a giant-magneto-resistive (GMR) or atunneling-magneto-resistive (TMR) sensor, which contains a magnetic freelayer 2, a non-magnetic spacer layer 3 and a magnetic reference layer 4.Spacer 3 is usually a conductive layer for GMR sensors and insulator forTMR sensors. Magnetization of reference layer 4, as represented byM_(reference), is fixed (i.e. ‘pinned’) in the X axis direction by anexchange field from other magnetic layers below it, which are not shownin the figure.

The pinned layer does not change its magnetization direction undernormal magnetic fields. In a conventional MR sensor, a bias DC fieldH_(bias) is usually applied in the Y direction by a pair of opposinghard magnets on the sides of the sensor, so that the free layer'smagnetization will be in the Y axis direction when no magnetic field isapplied. However, this free layer alignment to Y axis can also beachieved by making the sensor dimension in the Y axis longer than in Xaxis due to the shape anisotropy. Because of the shape anisotropy, themagnetization of the free layer 2, as represented by M_(free), can onlyrotate freely in the XY plane when a transverse field is applied alongthe X axis direction and it is very difficult to rotate outside the XYplane, i.e. towards the Z axis.

If a magnetic field is applied in the X axis direction, the free layermagnetization rotates away from the Y axis and the resistance of theentire MR junction will change according toR=R ₀ −ΔR cos θ

where R₀ is the base resistance of the sensor, ΔR is the full rangeresistance change of the sensor and θ is the angle between themagnetization of the reference layer and the free layer. With a DCcurrent applied to the device, where the current can either flow in theXY plane or perpendicularly through the device, the voltage across thedevice will change, because of the resistance change, to produce ameasurable voltage signal.

In prior studies and patents, several detection schemes were used. Onecommonly used scheme is to apply a magnetic field in the transversedirection [4-10, 12-13], e.g. along the X axis as in FIG. 1.Super-paramagnetic beads are also used. When a super-paramagnetic beadis bound to the top surface of the MR sensor, this applied field canmagnetize the bead magnetization along the field direction. The beadmagnetic moment in turn will produce a magnetic field in the MR sensorbelow and partially cancel the original applied magnetic field acting onthe MR sensor. Therefore, the voltage across the sensor when a bead ispresent is different than when there is no bead attached at the sameapplied field condition and the presence of the bead is detected by thisvoltage amplitude difference.

It is important to note that these prior art methods use appliedmagnetic fields oscillating at frequencies less than 100 kHz (often muchless) whereas the present invention requires frequencies in the MHZ (andhigher) region.

A reference sensor to which beads will not attach to at any time isnormally used for comparison of this voltage difference. During thedetection, the applied field can also be a modulated by an AC field thatwill induce a same frequency AC voltage across the sensor. By utilizinga lock-in technique, the signal to noise ratio can be enhanced.

Another bead sensing scheme, also known as BARC [1-3,11], is to apply aDC field perpendicular to the film plane, i.e. along the Z axisdirection in FIG. 1, with no bias field at all being present. This DCfield serves to magnetize the super-paramagnetic bead moment vertically.The in plane component of the field generated by the vertical beadmoment will rotate the free layer magnetizations in the upper XY planeand low XY plane towards or away from the Y axis at the same time. Ifthe reference layer magnetization is aligned along the Y axis, or amulti-layer MR structure is used, this rotation will produce aresistance change. It is also called “scissoring mode” [11]. This schemealso needs a reference sensor for detection.

A potential problem common to all the previously published or patentedbead-MR sensor detection methods is that they are prone to fluctuationsin the magnetic signal. Since all these detection methods are mainlypracticed in the low frequency region from several Hertz to severalkHertz, 1/f noise is very significant. Although the lock-in techniquecan successfully suppress the noise from the electrical sources by itsnarrow bandwidth, the noise from magnetic sources, for exampleBarkhausen noise, popcorn noise and telegraph noise cannot be preventedfrom affecting the locked-in signal. These magnetic noise sources arerelated to domain and local magnetization switching of MR sensors andare always most predominant in the low frequency region and usually showup as signal level random fluctuations. For the scheme that requires areference sensor for detection, this combined noise effect from both thedetection sensor and the reference sensor will at least double thisparasitic fluctuation of the signal level.

Besides the fluctuations from the sensor, the bead itself will alwayshave shape, size and magnetic content variations as well as binding sitevariations. These variations will also cause fluctuations of the amountof the bead magnetic field going into the sensor. With all the noisesources added together, these can be quite large and can causesignificant signal fluctuations to inhibit practical binding assayapplications that are based on the detection of the absolute fieldstrength.

Another problem specifically for the field cancellation method is that amagnetic field needs to be applied in the sensing direction, i.e. Xaxis, to magnetize the magnetic beads and thus generate the cancellationfield. However, this relatively large amplitude field will rotate the MRsensor free layer magnetization to the place where its sensitivity isnot the highest. In other words, when the bead field is highest, thesensor sensitivity is lowest. By proper design of the MR sensor filmstructure and by using a vertical AC field to mimic the scissoring mode,second harmonic detection [7] can minimize the sensitivity loss. But thesignal generated by the cancellation effect from the bead field issignificantly decreased because it is only operating with a single beadfield polarity and not a full reversal of the bead field direction inthe transverse direction.

For the scissoring mode, where the bead is magnetized vertically, thereis no sensitivity concern. However, this mode requires a relativelylarge sensor size. For a sub-micron or deep submicron size sensor, theexchange energy within the free layer will degrade the amount ofrotation achievable for the two regions of the sensor rotating againsteach other. A low bias field, H_(bias), or no bias field may be needed,which can easily lead to instability in a micron size sensor because ofweak or no free layer domain control. In addition, for a GMR or TMRsensor with a single free layer, since this scheme only utilizes thesensor free layer magnetization rotation between 0 and 90 degrees, halfof the sensor sensitivity region is not used.

For a multilayer GMR sensor, the rotation of magnetization cantheoretically reach maximum or 0 to 180 degrees. A current-in-plane(CIP) multilayer GMR sensor usually has a lower dR/R, i.e. lower signal,than the state-of-the-art TMR or carefully designed spin valve GMRsensors. The current-perpendicular-to-plane (CPP) multilayer GMR sensorsalthough possessing a much higher dR/R than the CIP ones, have alsoshown extraordinary magnetically related 1/f type noise in previousstudies.

This noise can be more than 10 dB over the sensor's Johnson noise levelin a micron size multilayer device and it will severely degrade the SNRof the sensor. To overcome the above problems, what is needed is, first,a scheme that can avoid having the bead magnetizing field affect thesensor free layer as well as utilizing the full reversal of the beadmagnetization to gain maximum signal. Second, to avoid fluctuation ofthe bead magnetic field or the sensor resistance due to various magneticsources, the method should not rely on detection of the absolute beadfield magnitude. Third, signal detection at much higher frequencies thancurrently being explored (<100 kHz), for example beyond 1 MHz, ispreferred in order to reduce low frequency noise effects.

A routine search of the prior art was performed with the followingreferences of interest being found:

-   [1] D. R. Baselt et al., “A biosensor based on magnetoresistance    technology,” Biosens. Bioelectron., vol. 13, pp. 731-739, October    1998.-   [2] R. L. Edelstein et al., “The BARC biosensor applied to the    detection of biological warfare agents,” Biosens. Bioelectron., vol.    14, pp. 805-813, January 2000.-   [3] M. M. Miller et al., “A DNA array sensor utilizing magnetic    microbeads and magnetoelectronic detection,” J. Magn. Magn. Mater.,    vol. 225, pp. 138-144, April 2001.-   [4] D. L. Graham, H. Ferreira, J. Bernardo, P. P. Freitas,    and J. M. S. Cabral, “Single magnetic microsphere placement and    detection on-chip using current line designs with integrated spin    valve sensors: Biotechnological applications,” J. Appl. Phys., vol.    91, pp. 7786-7788, May 2002.-   [5] H. Ferreira, D. L. Graham, P. P. Freitas, and J. M. S. Cabral,    “Biodetection using magnetically labeled biomolecules and    arrays,” J. Appl. Phys., vol. 93, pp. 7281, May 2003.-   [6] G. Li et al., “Detection of single micron-sized magnetic bead    and magnetic nanoparticles using spin valve sensors for biological    applications,” J. Appl. Phys., vol. 93, pp. 7557-7559, May 2003.-   [7] G. Li, S. X. Wang and S. Sun, “Model and experiment of detecting    multiple magnetic nanoparticles as biomolecular labels by spin valve    sesnors,” IEEE Trans. Magn., vol. 40, pp. 3000, 2004-   [8] S. X. Wang et al., “Towards a magnetic microarray for sensitive    diagnostics,” J. Magn. Magn. Mater., vol. 293, pp. 731-736, 2005.-   [9] W. Shen, X. Liu, D. Mazumdar and G. Xiao, “In situ detection of    single micron-sized magnetic beads using magnetic,” Appl. Phys.    Lett., vol. 86, pp. 253901, 2005.-   [10] H. Ferreira, N. Feliciano, D. L. Graham and P. P. Freitas,    “Effect of spin-valve sensor magnetostatic fields on nanobead    detection,” J. Appl. Phys., vol. 97, pp. 10Q904, 2005.-   [11] D. R. Baselt, “Biosensor using magnetically detected label,”    U.S. Pat. No. 5,981,297 (1999) teaches that a change in output of MR    sensors indicates the presence of magnetic particles.-   [12] M. C. Tondra, “Magnetizable Bead Detector,” U.S. Pat. No.    6,743,639 B1 (2004)-   [13] M. C. Tondra, “Magnetizable Bead Detector,” U.S. Pat. No.    6,875,621 B2 (2005); this, and ref. 12 above, shows an MR sensor in    a bridge circuit which may comprise interconnected individual    sensors adjacent to the binding molecule layer.-   [14] U.S. Pat. No. 6,518,747 (Sager et al) discloses applying an AC    signal to excite Hall sensors in a DC field to detect magnetic    particles.-   [15] U.S. Patent Application 2006/0020371 (Ham et al) discusses FMR    detection of magnetic beads.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the presentinvention to provide a sensor for the detection of magnetic beadstogether with a method for utilizing said sensor.

Another object of at least one embodiment of the present invention hasbeen that said sensor be relatively insensitive to the precise locationof a bead relative to the sensor as well as to the exact size of thebead.

Still another object of at least one embodiment of the present inventionhas been that said sensor be relatively insensitive to the presence ofother nearby beads.

A further object of at least one embodiment of the present invention hasbeen that said method not require the measurement of the absolute valueof a magnetic field generated by the bead.

These objects have been achieved by exciting FMR (ferromagneticresonance) in the bead and then detecting the rotating magnetic fieldthat the bead emits while in the resonance state. Our preferred meansfor detecting said rotating field has been a magneto-resistive detectorsuch as a GMR (giant magnetic resistance) or TMJ (tunneling magneticjunction) device, though the method can be effectively used with anydevice capable of detecting magnetic fields as low as 1 Oe that areoscillating at frequencies in the MHz to GHz range. As a practicalmatter, the lowest field that can be used will be determined by theintrinsic noise discrimination and detection sensitivity of the sensor.Additionally, the detection sensitivity can usually be improved bynarrowing the band width of the signal that is being detected.

The oscillating magnetic field used to excite the bead to FMR may begenerated in a number of ways including external means and in-situgeneration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art magnetic bead detector.

FIG. 2 illustrates the basic method of the invention, including thevarious magnetic fields that are needed to bring about FMR.

FIG. 3 is FIG. 2 viewed along the Y direction.

FIG. 4 plots the X component of the bead magnetization vs. time afterthe AC field is applied in the Y axis direction for three differentvalues of h_(AC).

FIGS. 5A-5D show four embodiments of the invention that have thefollowing features in common: H_(DC) in the Z direction and h_(AC) inthe Y direction.

FIGS. 6A-6F show five embodiments of the invention that have thefollowing features in common: H_(DC) in the Z direction and h_(AC) inthe X direction.

FIGS. 7A-7D show four embodiments of the invention that have thefollowing features in common: H_(DC) in the Y direction and h_(AC) inthe Z direction.

FIGS. 7E-7F show two embodiments similar to FIGS. 7A and 7Brespectively, but with H_(bias) provided by external magnets as in FIG.5A.

FIGS. 8A-8B are similar to FIGS. 5 thru 6 and FIG. 7, respectively,except that the overlapping field generating stripes are not required tobe orthogonal to one another.

FIG. 9 illustrates a method for determining the spatial location of anyparticular bead that is part of an array of beads.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the detection schemes to be described below, a uniform magnetizationof the magnetic bead is assumed. The bead is only required to have amagnetic moment, either spontaneous or induced, when in the field. FIG.2A shows a schematic view of the detection of FMR in an excited magneticbead by a MR sensor. Free layer 2 of the MR sensor is in the XY plane. AY axis direction DC field H_(bias) (applied or anisotropy driven) in thesensor serves to maintain the free layer magnetization along the Y axisdirection when there is no transverse X axis field applied. Note that,although we refer to a free layer when we describe the invention, we doso as a matter of convenience rather than to exclude other methods fordetecting magnetic fields such as normal (i.e. not giant)magnetoresistance.

Magnetic bead 1 lies above the top surface of the sensor. An externallyapplied static field H_(DC) perpendicular to the sensor film planeorients the bead magnetization in the Z axis direction. Since the sensoronly responds to X axis direction transverse fields, this DC field willnot cause sensor resistance to change. Then, a low amplitude, highfrequency, sinusoidal AC field h_(AC) is applied in the Y axis directionto induce ferromagnetic resonance of the bead magnetization. This Y axissinusoidal field does not itself cause the free layer magnetization torotate as long as it is much smaller than H_(bias).

Basis of the Invention:

In FIG. 2 we illustrate the basic effect of FMR on the magnetic momentof a bead excited by an AC magnetic field h_(AC) in the presence of a DCmagnetic field H_(DC). MR sensor magnetization is oriented in the Y axisdirection by a bias DC field H_(bias). The magnetic field generated bythe resonating bead's magnetic moment is sensed by the MR sensor's freelayer below. The X component of the resonating bead's moment generates atransverse field in the sensor free layer which leads to an AC voltageacross the sensor at the FMR frequency.

As shown in FIG. 3, this causes the free layer magnetization to rotate.Since m_(x) has the same frequency, the free layer magnetization will bemodulated at this frequency by the bead field. This modulation furtherleads to a voltage alternating at the same frequency as that of thesensor (which usually implies the free layer). Thus, by tuning thestructure so as to achieve resonance, one can be certain that anymodulation of the sensor's output derives from the bead(s).

FIG. 4 shows simulation of the X component of the magnetic bead's momentas a function of time for the detection structure shown in FIG. 2.H_(DC) for the simulation was 100 Oe, a damping constant of about 0.02was used, with the Y axis AC field (h_(AC)) frequency being that of thebead resonance frequency ω_(r), for a 100 Oe field (279.73 MHz). Resultsfor three different values of h_(AC) are shown. Curve 41 is for 1 Oe,curve 42 is for 2 Oe, and curve 43 is for 5 Oe. These show that, for a0.02 damping constant, a 5 Oe AC field is already enough to generate afull amplitude resonance of the bead moment, i.e. m_(x)=m_(bead), at thegiven condition.

In first order calculations, the super-paramagnetic effect is notconsidered. For super-paramagnetic beads, a h_(AC) field beyond 10 Oemay not be trivial for an H_(DC)=50 Oe or 100 Oe. The AC field can alsocause the bead moment to vary in magnitude instead of just direction.

Besides the comparison of the oscillation amplitude of bead moment atdifferent frequencies or different DC fields, another way to detect thepresence of a magnetic bead is to check the existence of an m_(x)component at an AC field near or at a bead's resonant frequency at thegiven DC field. When there is no bead, no m_(x) component is present.This method is an absolute signal level detection method so it issusceptible to bead field strength variations from various sources as inthe prior art. However, it still has the advantage of detection at thebead FMR that produces the largest m_(x) amplitude, i.e. highestdetectable signal, using a relatively small excitation AC field.

The m_(x) component of the bead magnetic moment will linearly influencethe sensor's resistance change. Several previous studies [7,10]demonstrated a straightforward way to estimate the signal level from aGMR or TMR sensor with a given magnetic bead that has a magnetic momentin the X axis direction. The MR signal from the bead moment was reportedto usually be a small portion of the entire dynamic range of the MRsensors, i.e. less than 1% to several percent of the dR/R. The signal ofthe bead FMR at the resonance frequency should be high enough to bepicked up by available RF electronics with an applicablesignal-to-noise-ratio (SNR). For example, in thermally excited FMR inGMR sensors, usually in the several GHz region and corresponding to lessthan 0.1% resistance fluctuation, has already been successfully measuredwith relatively simple RF circuitry [25]. Therefore, FMR detectionschemes proposed for monitoring the m_(x) component of the bead magneticmoment should be achievable by measuring the MR sensor voltage output.

In summary, use of FMR to detect the presence of beads offers thefollowing advantages:

1. The MR sensor responds to only the bead field during detection. TheDC field for magnetizing super-paramagnetic beads and the AC excitationfield can be applied in other than the sensing direction of the MRsensor. Thus there is no tradeoff needed between sensitivity and beadmagnetic signal. In addition, it allows for adjustment of the sensorbias field towards lower values to enhance the sensor sensitivity totransverse fields.2. For a bead of low damping material, only a relatively small AC fieldis needed to excite full amplitude resonance in the bead so as toproduce the maximum AC field in the sensing direction of the MR sensor.3. The method is minimally affected by low frequency parasitic magneticfluctuations. The excited FMR of the bead magnetic moment, undermoderate magnetic fields. is usually at a frequency beyond theBarkhausen noise, popcorn noise, telegraph noise, and 1/f noise activeregions. For example, the FMR frequency at a DC field of 100 Oe is about280 MHz.4. A reference sensor is not needed for signal detection. Measurement ofthe relative amplitude or phase of the FMR signal at different AC fieldfrequencies at a constant DC field, or at the same AC field frequencybut varying DC fields, will indicate the presence of a magnetic bead.For example, by keeping the DC and AC field amplitudes constant whilesweeping the AC field frequency from a value lower than ω_(r) to afrequency higher than ω_(r.), while simultaneously measuring the sensorsignal at the same frequency. When a magnetic bead is present the sensorsignal peaks at ω_(r) but if there is no bead, the signal vs. AC fieldfrequency will stay flat. Peak detection of the swept curve can thus beused as an indicator of a bead's presence.5. The method is insensitive to the precise location of the binding siteof the bead on the MR sensor. In absolute field level detection methodsthe magnetic field level that the sensor sees (due to the bead) will bedifferent for different binding sites. However, as discussed in thepreceding paragraph, the frequency dependence of the signal on the ACfield frequency will stay the same for a given bead. Therefore, ifdetection is achieved by sweeping the AC field frequency, local bindingsite variations will not affect the peaking of the AC signal at ω_(r).6. The method is relatively insensitive to bead size distribution. Fieldcancellation methods are strongly affected by the beads' physical sizedistribution because each bead's magnetic moment is proportional to itsvolume. With a large bead size distribution, the magnetic field from thebeads will fluctuate substantially. For FMR detection, however, althoughbead size variations will still cause the absolute signal level tofluctuate, as long as the beads' composition and shape are the same, thefrequency dependence of the AC signal from each bead still stays thesame. As long as this frequency dependence is used for detecting thebeads' presence (and assuming sufficient bead field strength) the schemethat is taught by this invention will be insensitive to the bead sizedistribution.7. The method is insensitive to the bead-sensor distance (see d in FIG.3). Longer biological or chemical binding pair lengths will lead tolonger distances between bead and sensor, which will decrease theabsolute level of the bead magnetic field at the sensor. However, forthe FMR detection scheme, as long as the bead-sensor distance is withina range that is low enough to show a dependence of the FMR signal on theAC field frequency, this physical distance variation can be tolerated.This insensitivity results in having greater flexibility when choosingbiological and chemical binding pairs.8. The method enables field effects from neighboring beads to beminimized. The AC field needed to evoke full amplitude FMR in a beadwith a relatively small damping constant is usually small (severalGauss). An AC field of this magnitude can be generated by passing an ACcurrent through a conductive stripe that runs beneath a row or column ofsensors or even by an individual line underneath each sensor, whichexcites only the beads directly above the stripe lines. Neighboringbeads not over the stripe line will not contribute to the signalgenerated by the sensor.

This feature may be used as the basis for a method to determine thespatial location of a single bead which may, or may not, be part of anarray of similar, or identical, beads. Se feature (g) below.

9. Since the resonance frequency is a function of the shape anisotropyof the magnetic beads and their damping constants, the methodfacilitates the simultaneous use of multiple beads whose resonancefrequencies are not necessarily all the same, even in the same DC field.Examples include, but are not limited to, elongated magnetic beads andbeads with a magnetic shell and a non-magnetic core. These structureswill have different FMR frequencies from a spherical magnetic beadbecause of the shape anisotropy. It is also possible to vary the dampingconstant of the bead through control of its composition in order toshift the FMR frequency. In addition, magnetic beads formed frommagnetic particles suspended in a nonmagnetic matrix will also exhibitdifferent FMR resonance behavior, depending on the densities of theparticles in the matrix.10. The method offers a way to quantify the number of beads attached toa given sensor from the resonance frequency shift caused by themagneto-static interaction between beads attached to the same sensor.From the FMR frequency shift of the ensemble of the beads, the numberand formation of the beads on the sensor surface is obtainable aftercareful characterization and calculation.Features of the Method

All necessary layers, coatings and structures that enable the MR sensorto function in the relevant biological or chemical environments areassumed in the embodiments.

The magnetic beads used in the embodiments vary in shape, structure andcomposition as needed to obtain different bead FMR frequencies under thesame DC field to enable bead labeling. The applied DC magnetic field inthe following embodiments is also the field used to magnetize the beadsshould they be super-paramagnetic.

The MR sensor for FMR detection is not limited to GMR and TMR sensors,but rather any thin film sensing device that can show a measurablechange in the presence of a magnetic field. The excited FMR of the beadmoment produces an AC magnetic field in the MR sensor and subsequentlyan AC voltage signal across the sensor when a sensing current isapplied. The detection of the presence of the bead can be achieved bythe following methods (list not intended to be exhaustive):

(a) Sensor signal amplitude dependence on the AC field frequency when DCfield is fixed. The amplitude is preferably measured at the samefrequency as the AC field.

(b) Sensor signal amplitude dependence on the DC field when AC fieldfrequency is fixed. The amplitude is preferably measured at the samefrequency as the AC field.

(c) Sensor signal phase dependence on the AC field frequency when DCfield is fixed. The phase should be measured at the same frequency asthe AC field.

(d) Sensor signal phase dependence on the DC field when AC fieldfrequency is fixed. The phase should be measured as close to the ACfield frequency as possible.

(e) The existence of an AC signal from the sensor at the frequency ofthe driving AC field, preferably near or at bead's resonant frequency.

(f) Cancellation of the driving AC field on the sensor at a frequencypreferably near or at bead's resonant frequency.

(g) Determining the precise location of a bead. In a conventional MRAM(magnetic random access memory), two non-parallel sets of conductivewires are used to form an array in which the intersection of any twowires (from opposing sets) is made to be unique by providing the sameinput to each wire, namely slightly more than half the magnetic fieldneeded to trigger a single device. Consequently, it is only at theintersection of the two wires that the local field becomes strong enoughto trigger a device.

In the present invention, the inputs provided to the two intersectingdiffer from one another. Referring now to FIG. 9, H_(DC) is provided byexternal means and I_(AC) (to generate h_(AC)) is sent down one of thelines which we will (arbitrarily) designate as line 92. A DC currentI_(DC) is sent down one of the non-parallel lines which we will(arbitrarily) designate as line 93, thereby generating a static magneticfield h_(DC) All beads on lines that run parallel to line 92 (e.g. line96) are powered so that they ‘see’ h_(AC) and H_(DC) while all beads onlines that run parallel to line 93 (e.g. line 94), except at theintersection, will see only H_(DC)+h_(DC) Thus, only bead 91, located atthe intersection, will also see h_(AC) so it will exhibit FMR, but at adifferent frequency from any beads lying along line 92, 96, etc.

Comments:

For methods (a) through (d), when a bead is present, the mentionedfrequency or DC field dependence is observed. When there is no bead, nodependence is seen.

For method (e), the absolute value of the bead field is detected i.e. ifthere is no bead there is no field. This method is susceptible to beadfield strength fluctuations caused by various sources. However, it stillhas the advantages that when detection is in the high frequency region,the MR sensor is at its maximum sensitivity and maximum bead magneticfield at a small FMR excitation field. Note that, for method (e) the ACfield is not to be applied in the sensing direction of the MR sensor.

For method (f), the absolute combined total field of the applied ACfield and the bead field is detected. When there is no bead, there is nobead field acting against the AC field effect on the sensor. This methodis thus similar to (e) and enjoys the same advantages as (e) over thefield cancellation methods of the prior art. In (f), however, the ACfield is applied in the sensing direction of MR device.

For method (g), H_(DC) is typically in the range of from about 1 and1,000 Oe while h_(DC) is typically in the range of from about 1 to 100Oe. For example, for H_(n)=100 Oe and h_(n)=20 Oe, FMR for beads on line1 will occur at about 280 MHz while for a bead at the intersection oflines 92 and 93 it will occur at about 285.6 MHz.

STRUCTURAL EMBODIMENTS OF THE INVENTION

Listed below are the preferred structural embodiments of this invention.Note that, although each of these depicts only a single bead over eachsensor, it is clear that extension to multiple beads per sensor isreadily implemented.

1. Embodiment 1A

Referring to FIG. 5A, the magnetic bead 1 is attached to the MR sensor 2through biological or chemical binding pair 5 following a recognitionprocess. Two permanent magnets 6 on the sides of the sensor provide abiasing field in the MR sensor to orient the sensing layer magnetizationin the Y axis direction. A static field H_(DC) is applied perpendicularto the sensor plane in the Z axis direction. This static fielddetermines the FMR frequency of the bead magnetic moment. It alsomagnetizes the bead when the bead is super-paramagnetic. A stripe line(or lines) 7 exists underneath the sensor or between the sensor and thebead, where an AC current I_(AC) is used to produce an AC magnetic fieldh_(AC) in the Y axis direction to excite the FMR of the bead magneticmoment. The bead FMR then produces a rotating component in the XY plane.The MR sensor 2 detects the oscillating magnetic field

2. Embodiment 1B

Referring to FIG. 5B, every other aspect is the same as embodiment 1Aexcept that the AC magnetic field h_(AC) in the Y axis direction isgenerated not by stripe line (or lines), but externally by other means.For example, RF coils or electromagnetic waves in a microwave cavity.

3. Embodiment 1C

Referring to FIG. 5C, every other aspect is the same as embodiment 1Aexcept that biasing DC field H_(bias) is applied externally or generatedby the anisotropy field of the sensor. H_(DC) (and H_(bias) as well ifapplied externally) determines the FMR frequency of the bead magneticmoment.

4. Embodiment 1D

Referring to FIG. 5D, every other aspect is the same as embodiment 1Cexcept that the AC magnetic field h_(AC) is generated not by stripe line(or lines), but externally by other means. For example, RF coils orelectromagnetic waves.

5. Embodiment 2A

Referring to FIG. 6A, every other aspect is the same as embodiment 1Aexcept that the stripe line (or lines) 7 that is underneath the sensoror between the sensor and the bead, is now used to produce an ACmagnetic field h_(AC) in the X direction to excite the FMR of the beadmagnetic moment when an AC current I_(AC) flows through it. The MRsensor 2 detects both h_(AC) and the field generated by the bead moment.The difference of the bead field cancellation of h_(AC) in the MR sensorat different driving frequencies can be used as the mechanism fordetection. The cancellation of h_(AC) is preferably measured in the samephase as h_(AC). Since m_(x) has the maximum amplitude and is the exactsame phase as h_(AC) when the frequency of h_(AC) is the same as the FMRfrequency of the magnetic bead in the DC field H_(DC), the dependence ofthe cancellation effect on the AC field frequency will be morepronounced when measured at the same phase as h_(AC)

6. Embodiment 2B

Referring to FIG. 6B, every other aspect is the same as embodiment 2Aexcept that the AC magnetic field h_(AC) in the X axis direction toexcite the FMR of the bead magnetic moment is generated not by a stripeline (or lines), but externally with other means. For example, RF coilsor electromagnetic waves.

7. Embodiment 2C

Referring to FIG. 6C, every other aspect is the same as embodiment 2Aexcept that the biasing DC field H_(bias) is applied externally orgenerated by the anisotropy field of the sensor. H_(DC) (and H_(bias) aswell if applied externally) determines the FMR frequency of the beadmagnetic moment.

8. Embodiment 2D

Referring to FIG. 6D. every other aspect is the same as embodiment 2Cexcept that the AC magnetic field h_(AC) in the X axis direction toexcite the FMR of the bead magnetic moment is generated not by a stripeline (or lines), but externally by other means. For example, coils orelectromagnetic waves.

9. Embodiment 2E

Referring to FIG. 6E. every other aspect is the same as embodiment 2Aexcept that the DC field H_(DC) is applied in the Y axis direction,which also serves as a biasing field to orient the sensor layermagnetization of the MR sensor in the Y axis direction.

10. Embodiment 2F

Referring to FIG. 6F every other aspect is the same as embodiment 2Eexcept that the AC magnetic field h_(AC) in the X axis direction toexcite the FMR of the bead magnetic moment is generated not by a stripeline (or lines), but externally by other means. For example, RF coils orelectromagnetic waves.

11. Embodiment 3A

Referring to FIG. 7A, bead 1 is attached to the MR sensor 2 throughbiological or chemical binding pair 5 after the recognition process. ADC field H_(DC) is applied in the sensor plane in the Y axis direction.H_(DC) determines the FMR frequency of the bead magnetic moment and canalso serve as a bias field to orient the sensor layer magnetization ofthe MR sensor in the Y axis direction. It also magnetizes the bead whenthe bead is super-paramagnetic. An AC magnetic field h_(ac) is appliedin the Z direction to excite the FMR of the bead magnetic moment. ThisAC field is generated by a pair of stripe lines 7 on the sides or overthe top or underneath the MR sensor. The AC currents flowing in thelines are constantly in the opposite direction to produce a net verticalAC field in the sensor. The MR sensor 2 detects the field generated bythe bead moment during the excited FMR.

12. Embodiment 3B

Referring to FIG. 7B, every other aspect is the same as embodiment 3Aexcept that the pair of stripe lines 7 that produce the AC magneticfield h_(AC) in the Z direction is now oriented in the X direction. As amatter of fact, this pair of lines can be oriented in any direction inthe XY plane as the Z axis field is not affected by their orientation.

13. Embodiment 3C

Referring to FIG. 7C, every other aspect is the same as embodiment 3Aexcept that the AC magnetic field h_(AC) in the Z axis direction toexcite the FMR of the bead magnetic moment is generated by coil 7(single or mufti turn) around, below or above the top of the sensor. TheAC current flowing in the coil produces a vertical AC field.

14. Embodiment 3D

Referring to FIG. 7D, every other aspect is the same as embodiment 3Aexcept that the AC magnetic field h_(AC) in the Z direction to excitethe FMR of the bead magnetic moment is applied externally with RF coilsor electromagnetic waves. This leads to a voltage signal of the samefrequency being generated by the sensor when there is a DC currentflowing through the sensor. Because m_(x) magnitude is a function offrequency, the voltage signal generated by the MR sensor at the same ACfield amplitude but different AC field frequencies will have differentoutput amplitudes as well. This amplitude dependence of the sensor'svoltage output on frequency can be used as the mechanism for thedetection of the presence of the magnetic beads. When a bead is notpresent, the sensor will theoretically have no output dependence on theAC field frequency because of the absence of the bead m_(x) component.

15. Embodiments 3E and 3F

Referring to FIGS. 7E and 7F, these are the same as embodiments 3A and3B respectively, except that the longitudinal bias is supplied bypermanents as shown, for example, in FIG. 5A.

16. Embodiments 4A and 4B

Referring to FIGS. 8A and 8B, these relate to embodiments 1A-2F and 3A,3B, 3E, 3F respectively but show that the field generating stripes andthe free layer's long axis do not have to be orthogonal to one anotherbut, rather, are required only to be as close to coplanar as theirthickness permits.

In addition, the phase of m, relative to h_(AC), will also showfrequency dependence. With the MR sensor free layer resistance closelyfollowing the field from the m_(x), this phase dependence on the h_(AC)frequency will also show up in the sensor's AC voltage signal and can beused as a detectable physical quantity as well.

1. A method to detect the presence of a magnetic bead, comprising:providing a detector that signals the presence of a magnetic fieldthrough a change in its electrical resistance, said detector having anupper surface that is parallel to a free layer of the detector; causingsaid free layer to be magnetized in a bias direction; attaching saidmagnetic bead to said upper surface at a binding site; providing astatic magnetic field, having a value, in a direction normal to saidupper surface; providing an alternating magnetic field having afrequency, a first amplitude, and a direction parallel to said uppersurface; and varying said frequency and said static field value untilferromagnetic resonance (FMR) occurs, whereby said magnetic bead emits amagnetic field that rotates at said FMR frequency at a second amplitude,thereby causing said detector signal to peak and to signal the presenceof said magnetic bead.
 2. The method of claim 1 wherein said directionof said provided alternating magnetic field is a first direction that isparallel to said bias direction or a second direction that isperpendicular to said first direction.
 3. The method of claim 1 whereinsaid magnetization in said bias direction is achieved through shapeanisotropy or through a magnetic field provided by a pair of permanentmagnets or by a pair of exchange-coupled magnets.
 4. The method of claim1 wherein said bead is ferromagnetic or super-paramagnetic orferrimagnetic.
 5. The method of claim 1 wherein FMR occurs at afrequency not lower than about 1 MHz, at which frequency parasiticeffects, including Barkhausen noise, popcorn noise, telegraph noise, and1/f noise are minimal, whereby magnetization along said bias directionmay be limited to generating a detectable signal of no more than 1 Oe,thereby increasing said detector's sensitivity.
 6. The method of claim 1wherein said magnetic bead has a damping constant of 0.2 or less,whereby said provided alternating magnetic field need be no greater thanabout 50 Oe, thereby increasing sensitivity of said sensor anddetectability of said bead.
 7. The method of claim 1 further comprisingcanceling any component of said provided alternating magnetic field inthe free layer whereby any AC signal generated by said sensor will beclosely related to FMR of said bead.
 8. The method of claim 1 whereinlocal binding site variations do not affect the peaking of the AC signalemitted by said sensor.
 9. The method of claim 1 wherein differences inrelative bead size in a range of about 0.01 to 1 micron will not affectthe peaking of the AC signal emitted by said sensor.
 10. The method ofclaim 1 wherein variations in bead distance from said upper surface thatare less than or equal to said bead's diameter will not affect thepeaking of the AC signal emitted by said sensor.
 11. The method of claim1 wherein said sensor is a member of an array of GMR or MTJ devices andthere is present a plurality of magnetic beads.
 12. The method of claim11 wherein FMR frequencies of at least two of said beads are differentfrom one another, said differences being due to variations in shape andcomposition of said beads, and then utilizing said differences in FMRfrequency to determine a spatial distribution of the beads.
 13. Themethod of claim 1 wherein said beads are used as magnetic labels in allapplications where successful detection of a label corresponds to thepresence of an object selected from the group consisting of molecules,chemicals, compounds, viruses, bacteria, and biological cells.
 14. Themethod of claim 1 wherein the presence of said magnetic bead is detectedthrough of a change of phase of said detector's output signal.